Many alloys with high weight percentages of a reactive metal, such as titanium, react with air and most common crucible refractories to the degree that the alloy is contaminated to an unacceptable extent. As a result, it common to melt such alloys in water cooled, metal (e.g. copper) crucibles using electric arc or induction to generate heat in the alloy charge for melting. U.S. Pat. Nos. 4,738,713 and 5,033,948 are representative of such melting techniques.
Alloys of titanium and aluminum forming intermetallic compounds, such as TiAl, have received considerable attention in recent years for use in the aerospace and automobile industries in service applications where their high strength at elevated temperature and relatively light weight are highly desireable. However, the intermetallic alloys contain a majority of titanium (e.g. so-called gamma TiAl includes 66 weight % Ti with the balance essentially Al) which makes melting and casting without contamination difficult and costly.
The Chandley and Flemings U.S. Pat. No. 5,299,619 describes an improved melting and casting technique for reactive metals and alloys, including those forming intermetallic compounds, wherein heating and melting of a charge of solid titanium in a ceramic crucible is accelerated by a robust exothermic reaction with a molten aluminum charge component that is separately melted and then introduced to the crucible to contact the titanium charge component. Reduced residence time of the melted charge components reduces potential contamination of the melt by reaction with the crucible materials.
Unfortunately, titanium based alloys, such Ti-6Al-4V, have insufficent aluminum present in the alloy composition to effect the robust exothermic reaction with titanium in the melting vessel for practicing the rapid melting, reduced contamination technique of U.S. Pat. No. 5,299,619. Since such "aluminum poor" titanium based alloys are in widespread use, there is a need for a melting method that can provide low cost, rapid melting of such "aluminum poor" reactive alloys with reduced contamination of the melt.
It is an object of the present invention to provide method and apparatus that satisfy the aforementioned need for a melting method and apparatus that can provide low cost, rapid melting, reduced contamination of "aluminum poor" titanium base alloys as well as other reactive alloys having compositions incapable of a robust exothermic reaction in a melting vessel.
It is another object of the present invention to provide method and apparatus for melting reactive metallic materials in a refractory melting vessel using selective and sequential induction heating of various solid metallic charge components segregated in a refractory crucible in a manner to effect top-to-bottom melting of the components that avoids harmful contamination of the melt.
It is another object of the present invention to provide method and apparatus useful for melting reactive metallic materials in a refractory melting vessel in top-to-bottom manner that eliminates the need for a separate melting step of one charge component that heretofore was melted first and then added to the vessel.